I am a fan of the "unsliced and unplanned"
Being "nervous" is part of the adventure. What you are embarking on is not a visit to Disneyland. You are supposed to be on edge. This is one of the fun parts!
I am all for pre-planning as a mental excercise, as a way to expand the notion of places and experiences that might be fun to "fit-into" a roadtrip. But I am ardent supporter of the notion of living an "unsliced life" and choosing a day's route solely on the weather, suggestions from fellow travelers and personal whims. There are a number of posters on this forum, (we appreciate their points of view too) that prefer to plan nearly every moment of each day of road trip -- but that is not my preference.
Your budget looks good. Booking a vehicle rental/lease from England will probably yield a better rate than doing it person. I have mentioned this before, but pick up a copy of "Drive USA" it is written from a Brit's point of view -- I don't agree with all of his suggestions -- but it can be helpful -- http://www.roadtripamerica.com/read/uk.htm
Jitters are a part of EVERYONE'S experience -- check out (http://www.roadtripamerica.com/roadtripamericaConference/
Index.cfm?CFApp=44&Message_ID=4243) for another new roadtripper's experience!
America is a big, friendly country. You already speak the language. Enjoy!
M.